Electronic Article Surveillance (“EAS”) systems are often used by retail stores in order to minimize loss due to theft. One common way to minimize retail theft is to attach a security tag to an article such that an unauthorized removal of the article can be detected. In some scenarios, a visual or audible alarm is generated based on such detection. For example, a security tag with an EAS element (e.g., an acousto-magnetic element) can be attached to an article offered for sale by a retail store. An EAS interrogation signal is transmitted at the entrance and/or exit of the retail store (“the interrogation zone”). The EAS interrogation signal causes the EAS element of the security tag to produce a detectable response if an attempt is made to remove the article without first detaching the security tag therefrom. The security tag must be detached from the article upon purchase thereof in order to prevent the visual or audible alarm from being generated.
One type of EAS security tag can include a tag body which engages a tack. The tack usually includes a tack head and a sharpened pin extending from the tack head. In use, the pin is inserted through the article to be protected. The shank or lower part of the pin is then locked within a cooperating aperture formed through the housing of the tag body. In some scenarios, the tag body may contain a Radio Frequency Identification (“RFID”) element or label. The RFID element can be interrogated by an RFID reader to obtain RFID data therefrom.
There are many known algorithms for determining the distance between a tag and another device or object (e.g., a tag reader or an EAS system pedestal defining an interrogation). One such algorithm is based on Received Signal Strength Indicator (“RSSI”) information. This RSSI based algorithm is hindered by the uncertainty of factors affecting the received signal strength such as loading of the antenna and the spatial orientation of one antenna to another antenna. In a passage system, it is desirable to quickly identify the presence of an EAS device containing an identification code within the interrogation zone. However, the scattering of Radio Frequency (“RF”) signals, loading effects, and spatial relation of one antenna to another introduces uncertainties as to the true location of the device.